The present invention relates to a solid composition having low magnetic signature which may be formed into a variety of shapes and which may have a variety of uses.
In underwater diving, it is customary to use metal weights to control buoyancy of the diver and his equipment. Divers use cast lead weights or pouches containing lead spheres, generally known as lead shot, to compensate for positive buoyancy. Marker weights and shots are another example of negative buoyancy devices used as accessories in diving operations. Life-support equipment for underwater diving often includes air-filled cavities whose positive buoyancy may be compensated for using metal weights which are often designed to form an inherent part of the system. Generally, high density metals exhibit the desirable characteristics for positive buoyancy compensation applications. Lead is frequently used to satisfy these requirements because it provides the most cost-effective weight-to-volume ratio.
In underwater diving in search of mines it is essential to reduce the total magnetic signature of the diver and his life-support system and accessory equipment to environmental levels. In absence of any electronic equipment, the total magnetic signature of a diver and his equipment includes two major components. Any magnetic materials such as ferromagnetic iron which may be contained in the system contribute one component of the signature. Eddy currents generated within conducting parts of the system moving in the Earth's magnetic field are responsible for the second component of the magnetic signature.
To optimize such a system in this regard it must be designed and constructed paying attention to the contributing aspects. For example some diver's weights have been made of purified lead with ferromagnetic impurities content below thresholds detectable in standard compositions tests. However, diver's weights made of lead or any other metal or alloy and totally free of ferromagnetic impurities, will still exhibit a finite magnetic signature when moving in a magnetic field, including the Earth's magnetic field. Eddy currents generated in the electrically conducting material produce a magnetic field that acts to oppose such motion. The magnitude of the generated magnetic field depends on the magnitude of the eddy currents which in turn depend on a number of factors. Those which are controlled by a designer of low magnetic signature equipment include the conductivity of the material, its size, physical composition and shape. The user and the manufacturer of the equipment should know how to handle the system to maintain the low magnetic signature characteristics. For example, he should recognize that magnetic impurities may enter the system through contact and friction with ferromagnetic materials or adhesion of environmental ferromagnetic dust, including asbestos particles.
A diver's weight made of solid lead constitutes a large conducting body in which significant eddy currents may be generated. Pouches of lead-shot also exhibit similar problems because lead spheres in contact with one another become a large conducting body with properties similar to those of the solid metal.